Method for electrochemical passivation of tinplate and electrolyte for use therein

ABSTRACT

A method for treating a material having a tin-containing surface to passivate and improve the corrosion resistance of the said surface which comprises making the material a cathode in an aqueous hexavalent chromium electrolyte consisting essentially of an aqueous solution of an alkali-metal dichromate in an amount of at least 20 and at most 25 grams per liter, an alkali-metal acetate in an amount of at least 5 and at most 10 grams per liter, and chromium trioxide in an amount of approximately 5 grams per liter, the said electrolyte having a hydrogen-ion concentration corresponding to a pH between 4.0 and 5.0 and an electrical conductivity of at least 28,000, preferably above 30,000, micromhos per centimeter at a temperature of 50* C, and passing an electric current through the material and the electrolyte until an amount of a chromium-containing film is deposited upon the said surface that is sufficient to improve the corrosion resistance of the said material.

Jargon et al.

[ METHOD FOR ELECTROCHEMICAL PASSIVATION OF TINPLATE AND ELECTROLYTE FORUSE THERElN [75] Inventors: Franz Jargon, Schwanewede; Erich Maschke,Bremen; Rudolf Eylens, Leuchtenberg, all of Germany [73] Assignee:Klockner-Werke AG, Duisburg,

Germany [22] Filed: May 2, 1974 [2l] Appl. No.: 466,247

[30] Foreign Application Priority Data May 9, i973 Germany 2323336 [52]US. Cl 204/56 R [51] Int. Cl. CZSD 11/38 [58] Field of Search 204/56 R;l06/l;

[56] References Cited UNITED STATES PATENTS 2,437,620 3/1948 Speer204/58 2,746,9l5 5/l956 Giesker et al 204/56 R 3.278,40l l0/l966 Smithet al 204/56 R Primary Examiner-R. L. Andrews Attorney, Agent. orFirm-Michael S. Striker [57} ABSTRACT A method for treating a materialhaving a tincontaining surface to passivate and improve the corrosionresistance of the said surface which comprises making the material acathode in an aqueous hexava lent chromium electrolyte consistingessentially of an aqueous solution of an alkali-metal dichromate in anamount of at least 20 and at most 25 grams per liter, an alkali-metalacetate in an amount of at least 5 and at most 10 grams per liter, andchromium trioxide in an amount of approximately 5 grams per liter, thesaid electrolyte having a hydrogen-ion concentration corresponding to apH between 4.0 and 5.0 and an electrical conductivity of at least28,000. preferably above 30,000, micromhos per centimeter at atemperature of 50 C, and passing an electric current through the material and the electrolyte until an amount of a chromium-containing filmis deposited upon the said surface that is sufficient to improve thecorrosion resistance of the said material.

8 Claims, N0 Drawings METHOD FOR ELECTROCHEMICAL PASSIVATION OF TINPLATEAND ELECTROLYTE FOR USE THEREIN INTRODUCTION The present inventionpertains to the electrochemical passivation of tinplate and similarmaterials having a tin-containing surface to improve their resistance tocorrosion so that they may be improved with respect to their suitabilityfor use for the production of containers for use in the canning andpackaging of foodstuffs. par ticularly protein-containing foodstuffs.

BACKGROUND OF THE INVENTION Tinplate is made by coating iron or steelsheet materials with a film of tin, either by dipping the sheet intomolten tin or by an electroplating operation. The amount of tin that isusually deposited in between 0.125 and 1.0 pound per base box (62,720square inches) which is equivalent to between l.40 and l l.2l grams persquare meter of the material. The thus coated sheet material has abright glossy surface.

In the absence of atmospheric oxygen or oxidizing agents, tin, unlikeiron, acts as an anode in the presence of organic acids such as arepresent in foodstuffs. Because of its higher hydrogen overvoltage (1.22volts compared to 0.82 volt for iron at 25C in 2-normal sulfuric acid atl amperes per square decimeter) the polarization of tin is changed.Because of this, tinplate is suitable for use as a packaging materialfor foodstuffs. It can be cold-worked, bent, and folded and sealed orwelded together and meat and other foodstuffs do not readily adherethereto.

Tinplate, however, has a tendency to stain by formation of tin sulfidewhen fish, meat, milk and certain vegetables containing proteins whichinclude a sulfur-containing amino acid component are placed into contacttherewith. Such staining is also referred to as marbling.

Conventional electrochemical aftertreatment of the tinplate in asuitable electrolyte offers one possibility of preventing such stainingby sulfur compounds. It is known that the resistance of tinplate tostaining from protein-containing foodstuffs can be increased bysubjecting the tinplate to an electrochemical aftertreatment in a sodiumdichromate-containing electrolyte, in which treatment the tinplate isused as a cathode. In such conventional treatments an electrolyte isused which consists of an aqueous solution containing be tween and 30grams per liter of sodium dichromate while the electrolyte is maintainedat a temperature between 7 l and 93C and its hydrogen-ion concentrationat a value corresponding to a pH between 3.5 and 5.5. In this process,the electric current density measured at the cathode that is applied isat least 3.76 and at most 21.5 amperes per square decimeter. Theduration of the treatment, that is, the period during which electricalcharges are passed from the tinplate cathode to the electrolyte isbetween 1 and 3 seconds, during which period the desired amount ofchromium and chromium oxide is deposited or plated on the tinplate.

As used herein, the term current density" is to be understood to referto the strength of the current per unit of cross-sectional area measuredat the cathode and is expressed in terms of amperes per squaredecimeter. The total amount of electrical current that is consumed orthe total electrical charge that is transferred to produce the specifiedchromium deposit on 2 the tinplate is specified herein in terms ofcoulombs (amperes per second) per square decimeter. The total amount ofelectrical current that is consumed is the product of the currentdensity and the period in seconds during which the current was passed.By use of the foregoing electrolyte and a temperature in the region of50C and a current density in the range between 0.2 and 0.5 ampere persquare decimeter, the amount of current used thus being between 0.2 and0.6 coulombs 0 per square decimeter, between about 3 and 5 milligrams ofchromium (as chromium and chromium oxide) per square meter will bedeposited upon the tinplate in accordance with this conventional method.By

increasing the current density to between 6.25 and 7.5 amperes persquare decimeter and thereby the amount of current to between 9.5 and lL25 coulombs per square decimeter, the amount of chromium and chromiumoxide that is deposited on the tin plate can be increased to between l5and l8 milligrams of chromium per square meter. By in creasing thetemperature of the electrolyte bath to about C, chromium and chromiumoxide in an amount equivalent to about 20 milligrams of chromium persquare meter will be deposited on the tinplate when the density of thecurrent is maintained at about 9.5 amperes per square decimeter, whichcorresponds to a total amount of 14 coulombs per square decimeter ofcurrent consumed.

The chromium coating deposited in accordance with this conventionalmethod will have not only a reduced tendency to stain upon contact withsulfur compounds but will also be less susceptible to etching orcorrosion when exposed to air or to acids or alkalies.

Despite such conventional electrochemical passivating treatments it hasbeen found that the tin coating on such tinplate sheets dissolves uponcontact with foodstuffs, unless the tinplate has been coated with anadditional protective film of lacquer or resin. Furthermore suchlacquer-coated tinplate is stained yellow at high temperatures which arenormally used for soldering and sealing the container made of suchtinplate. This staining is also referred to as scorch discoloration. Consequently the utility of such tinplate is also limited by thestability of the coating to heat.

A method is disclosed in US. Pat. No. 3,278,401 for improving thecorrosion resistance of tin-containing surfaces and especially surfacesof tinplate on iron or steel substrates coated with iron-tin alloys bycathodically depositing chromium thereon. The electrolyte that is usedin that method consists of an aqueous solution of a water-solublechromate and sodium acetate or other water-soluble acetate. The sodiumdichromate of that electrolyte was said to be replaceable by chromicacid (chromium trioxide CrO and other water-soluble chromates. Onlyconventional weights of the coatings of chromium and chromium oxide wereapplied in this manner to the tin-coated iron or steel substrate. Inthis patent it was stated that the chromium deposit had good lacqueradhesion properties. Consequently it is evident that a film of lacquerwas intended to be applied over the chromium coating to avoid thedissolving or migration of larger portions of tin from the thustreatedsurface of the tinplate. The application of a film of lacquer increasesconsiderably the cost of such tinplate. In that process the density ofthe electrical cur rent that was applied was also limited andconsequently the amount of chromium that could be deposited and theextent of the passivation that could be produced was alsocorrespondingly restricted.

Another process is disclosed in U.S. Pat. No. 3,49l ,001 for theelectrochemical treatment of tinplate to increase its corrosionresistance by means of a preliminary anodic treatment with a solution ofammonium or an alkali-metal carbonate followed by a cathodic treatmentin an aqueous solution containing sodium dichromate having ahydrogen-ion concentration corresponding to a pH of less than 2.0. Thistreatment produced deposits of chromium having only the con' ventionalweight or thickness which had the same disadvantages as the priortinplate even though the treatment was said to provide an improved basefor lacquer and other organinc coatings. Because of the highhydrogen-ion concentration, as represented by the low pH that is used inthat treatment, the lining of the tank in which the electrochemicaltreatment is conducted is highly etched or corroded.

A process is disclosed in German Democratic Republic Pat. No. 45,536 inwhich corrosion and staining by sulfur compounds is said to be inhibitedby the use of an electrolyte consisting of an aqueous solution of analkali-metal dichromate having a hydrogen-ion concentrationcorresponding to a pH between 4.0 and 6.0. In accordance with thatprocess deposits of chromium having only the normal weight or thicknesswhich had been obtained in other processes and which have the same knowndisadvantages that other deposits having such weights could be obtained.

SUMMARY OF THE INVENTION The object of the present invention is toprovide a method and an electrolyte for the production of passivatedtinplate that does not have the foregoing disadvantages, from whichtinplate only relatively insignificant amounts of the tin coating aredissolved and which coatings also have a high stability to heat.

A further object of the present invention is to provide a method andelectrolyte for passivating tinplate in which method the lining of thetank in which the electrochemical treatment is conducted is subjected toless destruction due to use.

The electrolyte that is used in the method of the present invention isdistinguished principally from those disclosed heretofore in containingat least 5 and at most grams per liter of an alkali-metal acetatetogether with 4 to 6, preferably 4.5 to 5.5, grams per liter of chromiumtrioxide (which is also referred to as chromic acid) the saidelectrolyte having a hydrogenion concentration corresponding to a pHbetween 4.0 and 5.0, and an electrical conductivity of at least 28,000,preferably at least 30,000, micromhos per centimeter at 50C. Theelectrolyte contains preferably at least and at most grams per liter ofan alkali-metal dichromate. Sodium acetate is an especially suitablealkalimetal acetate for use in the electrolyte.

The method of the present invention is distinguished principally fromthose disclosed heretofore in that the aftertreatment of the tinplate orsheets thereof that are to be used for the production of containers orcans, particularly those that are to be used for the packaging offoodstuffs, in the use of an electric current density measured at thecathode of at least 5.5 and at most 9.5 amperes per square decimeter,during a period of between l and 2 seconds, which corresponds to aconsumption of electric current amounting to from 8 to 14 coulombs persquare decimeter.

The aqueous electrolyte bath is preferably prepared by use of distilledor deionized water and is maintained 4 during the electrochemicaltreatment at a temperature between 30 and C, and preferably between 40and 50C.

By observing the foregoing parameters, deposits of chromium and chromiumoxides amounting to between 25 and 35 milligrams of chromium per squaremeter upon tinplate can be obtained by use of relatively low electriccurrent densities and without maintaining the electrolyte at highertemperatures.

Tinplate thus passivated in accordance with the method of the presentinvention has a higher resistance to staining by sulfur com pounds andits susceptibility to atmospheric corrosion and corrosion in acidic andalkaline media is substantially reduced. The method of the presentinvention can be adapted for use in existing electrolytic tinplatemanufacturing plants without re quiring substantial alterations of theplant when the electrolyte of the present invention is used. A muchsmaller electric current density is required when the electrolyte of thepresent invention is used in place of the prior electrolyte that wasused to deposit conventional coatings of normal thickness of chromiumand chromium oxide to tinplate without changing the length of the periodof treatment.

It is essential in order to increase the amount of deposited chromiumand chromium oxide on tinplate in accordance with the method of thepresent invention that the electrical conductivity of the electrolyte beincreased from the values in the region of 20.000 micromhos percentimeter that were previously used to values of preferably at least30,000 micromhos per centimeter and by the use of chromium trioxide andthe specified additional amount of alkali-metal acetate.

Tests extending for periods up to more than six months reveal that lessthan 60 milligrams of tin per kilogram of foodstuff dissolve or migratein protein-containing foodstuffs that are packaged in containers madefrom tinplate passivated in accordance with the method of the presentinvention. Furthermore, such tinplate has a high resistance tocorrosion, heat, and marbling.

The method and electrolyte of the present invention are illustrated anddescribed further in the detailed description which follows.

DETAILED DESCRIPTION In the following examples the passivating layersobtained consist of chromium, chromium trioxide and other chromiumcompounds, however, only the chromium content of the passivating layersis indicated.

EXAMPLE 1 A solution was prepared by dissolving the following substancesin distilled water in the amounts specified: 25 grams per liter ofsodium dichromate (Na Cr O 5 grams per liter of chromium trioxide (CrOand 10 grams per liter of sodium acetate (CH COONa).

The resulting solution had a hydrogen-ion concentration corresponding toa pH value of 4.6 and it had an electrical conductivity of 34,000micromhos per centimeter at 50C.

A continuous strip of tinplate both sides of which had been coated withtin having a width of 800 millimeters was connected to a source ofelectrical potential so that it could act as a cathode in anelectrolytic reaction. This sheet was continuously passed between twopairs of anodes that were installed in a passivation tank containing theforegoing electrolyte, each anode having a length of l500 millimeters.These anodes were so arranged that one pair of anodes was above and theother pair of anodes was below the tinplate sheet as it passed throughthe tank so that the sheet passed between the two pairs of anodes for atotal distance of 3000 millimeters. The sheet was passed between the twopairs of anodes at a linear speed of 2 meters per second so that theperiod during which electrical charges were actually transferred fromthe cathode to the electrolyte was l.5 seconds and the total surfacearea of the sheet, both sides of which were thus subjected to treatment,amounted to 480 square decimeters.

The temperature at which the electrolyte was maintained during theelectrolysis was 50C.

The total amount of electrical current per unit of surface area that wasconsumed by the tinplate sheet in this passivating tank was varied asspecified in the following table. The total amounts of chromium inmilligrams per square meter (rounded off to the nearest integer) thatwere thus deposited on both sides of the sheet are listed opposite theamount of the total amount of electrical current that was consumed incoulombs per unit of surface area, expressed as coulombs per squaredecimeter, in this table.

Total electrical current consumed coulombs per square decimeter Chromiumdeposited. milligrams per square meter COMPARATIVE EXAMPLE I In thisexample a solution corresponding essentially to that specified inExample I of U.S. Pat. No. 3,278,40l was used. This solution consistedof 25 grams per liter of sodium dichromate (Na Cr O-,),

and 2.25 grams per liter of sodium acetate (CH COONa) dissolved indistilled water. It had a hydrogen-ion concentration corresponding to apH of 5.3 and an electrical conductivity of 24,000 micromhos at 50C. Thecontinuous tinplate sheet that was used in Example I hereinbefore wasthen treated exactly as described in Example I while the temperature ofthe electrolyte was maintained at the same temperature that was used inExample I, with the following results:

Total electrical current consumed Chromium deposited millicoulombs persquare decimeter grams per square meter 0. l 5 l 0.30 l-2 0.65 2-3 1.304 I95 6 3.22 X 4 0 HI COMPARATIVE EXAMPLE 2 In this example a solutioncontaining the maximum proportion of sodium acetate that is specified inthe US. Pat. No. 3.278.401 was used. This solution con sistcd ol 25grams per liter of sodium dichromate (Na Cr O and 3.75 grams per literof sodium acetate (CH COONa) dissolved in distilled water. It had ahydrogen ion con centration corresponding to a pH of 5.4 and anelectrical conductivity of 26,000 micromhos at 50C. The continuoustinplate sheet that was used in Example I hereinbefore was treated inexactly the same manner as described in Example I while the electrolytewas maintained at the same temperature that was specified in Example I,with the following results:

Chromium deposited milligrams per square meter Total electrical currentconsumed coulombs per square decimeter COMPARATIVE EXAMPLE 3 Anelectrolyte consisting of a solution of 25 grams per liter of sodiumdichromate (Na- Cr O in distilled water having a hydrogen-ionconcentration corresponding to a pH of 3.8 and an electricalconductivity of 22,700 micromhos per centimeter at 50C was used in thisexample.

When the continuous sheet of tinplate that was used in Example I wastreated with this solution in the same manner as described in Example Iwith the electrolyte at the same temperature that was maintained inExample l, and with a current density of 0.21 ampere per squaredecimeter for a period of 1.5 seconds, corresponding to a total amountof electrical current consumed of 0.3l coulomb per square decimeter, thetotal amount of chromium deposited on both sides of the sheet wasbetween 3 and 5 milligrams per square meter (conventional method).

When the current density was increased to 5.0 amperes per squaredecimeter, corresponding to a total amount of electrical currentconsumed of 7.5 coulombs per square decimeter, the total amount ofchromium deposited on both sides of the sheet was between l4 and 16milligrams per square meter.

Because of the low conductivity of the electrolyte of the comparativeexamples I to 3 it is not possible to use a higher current density. andbecause of the small amount of chromium deposited. a higher degree ofpassivity cannot be obtained with such electrolytes.

Example I together with comparative examples I to 3 demonstrate that theelectrolyte that is used in the process of the present inventionproduces a substantially greater deposit of chromium in comparison with7 8 known electrolytes at both low and high current densicominued ties.The passivating layers that are obtained with the Chromium Tin Ironelectrolyte disclosed in U.S. Pat. No. 3,278,401 can 4 h :6 187 55 alsobe obtained with electrolytes containing only an 5 1:6 21:0 6:0alkali-metal dichromate with no addition of sodium ac- 5 6 months 3 7.5

etate.

The same results that are described in Example 1 and the ComparativeExamples 1 to 3 are obtained when this respect such highly passivatedhhplate also deionized water is used instead of distilled water. hibltshigh stability to marbling as as high resls' 1O tance to corrosion, evenin sterilization experiments EXAMPLE 2 toward media that simulate foodswhich contain such To t bli h h amounts of ri chromium and ironsubstances as table salt, acetic acid, lactic acid, thioglythat migrateinto canned meats from tinplate coated cohc acid and mixtures thereof attemperatures of with chromium passivating layers, pork and corned 121CfQT Periods between 30 and 60 mihutesy beef were packaged in cans formedfrom a commercial also have a greatly improved resistance to staining ytinplate coated on both sides known as tin plate No. 57 sulfurcompounds- 175 b hardness Temper 3 the chromium Without furtheranalysis, the foregoing will so fully sivating layers of which wereequivalent to deposi f reveal the gist of the present invention thatothers can, 4 and 25 milligrams of chromium per square meter, reypplying Cu ren knowledge, readily adapt it for varspectively, Theresults are listed in the following table ious applications withoutOmimhg features that from in which 1 refers to the tinplate having achromium the standpoint of Prior art, constitute essential layerequivalent to 4 milligrams per square meter and ll FhaYaQtel'istiCs ofthe generic or specific aspects of this refers to the tinplate having achromium layer equivalhvehhoh lent to milligrams per square meter, Whaatis claimed as new and desired to be protected The amounts of the metalsthat were dissolved or mi 25 y Patent lsi grated are tabulated in unitsof milligrams of the metal A method for g a material having a perkilogram of the meat. The amounts of metals that talhihg surface topasslvate and improve the corrosion dissolved were determined at thefollowing intervals, resistance of the said surface which comprisesmaking which are listed as the headings of the five columns of thematerial a Cathode in an aqueous hexavaleht chr0- these tables under therespective letter designations: mihm electrolyte Consisting essentiallyOf an aqueous A; Dire tl aft t ili ti solution of an alkali-metaldichromate in an amount of B; After st ra f r 1 month at roomtemperature at least 20 and at most 25 grams per liter, an alkali- C:After storage for 2 m ths t r temperature metal acetate in an amount ofat least 5 and at most 10 D; After t r f 1 month at 37C grams per liter,and chromium trioxide in an amount of E; Af Sim-age f 2 months at 37C atleast 4 and at most 6 grams per liter, the said electro- Pork A B D E 111 1 11 1 11 1 11 1 11 Iron 15.3 8.0 17 0 8.5 21.0 10.5 24.0 12.0 24.5140 Tin 0 0 5.0 9 0 5.5 13.0 8.0 17.0 8.5 19.0 10.0 Chromium 1 r 1,4 171.4 1.8 1.4 1.8 1.8 1.8 1.8

Corned Beef A B c o E 1 11 1 11 1 11 1 11 1 11 Iron 30.0 24.0 33.0 28.031510 32.0 38.0 27.0 38.5 33,0 Tin 5.2 5.2 9.0 7.5 10.5 8.6 185 10.023.0 12.0 Chromium 1.4 1.7 1.6 1.8 1.7 1.8 1 s 1.8 L8 1.8

The quantities of metals which migrated from cans made from tinplate inwhich evaporated milk having a butterfat content of 7.5% was packagedwere also dely havlhg a hydrogeh'loh concentration Correspondtermined.The cans were made of the same tinplate as mg P f P between and and anelectrical conspecified hereinbefore in connection with the cannedduchvhy of at least 28,000 mlcromhos P Cehhmeter meats that had achromium passivating layer equivalent at a emperahlre of and Passmg anelectric to 25 illi f h i per Square meter Th rent through the materialand the electrolyte until an amounts of the respective metals thatmigrated into the amouht of chromhlmjcohtalhmg h is deposited pevaporated milk packaged in such cans in milligrams the saldsurface that1s sufiiclentto improve the corroper kilogram of the milk after storageat room temperareslstahce 0f the sfildmfiteflalture between 22 and 27Cfor the specified periods 2. A method as defined 1n claim 1 1n which theelecare [iswd i h f n i l tric current is passed through the materialand the electrolyte for a period between l and 2 seconds and theChromium Tin 1m said current has a density as measured at the cathode ofat least 5.5 and at most 9.5 amperes per square decimei :33 g g ter, sothat between 8 and 14 coulombs per square dec- 5 months 1:6 17:: 5 0imeter of current are consumed.

3. A method as defined in claim 2 in which the electrolyte is maintainedat a temperature between 30 and 60C.

4. A method as defined in claim 2 in which the electrolyte is maintainedat a temperature between 40 and 50C.

5. A method as defined in claim 2 in which the electric current ispassed until the amount of chromiumcontaining film that is added to thematerial is equivalent to at least 25 milligrams of chromium per squaremeter of material.

6. A method as defined in claim 1 in which the alkalimetal acetate issodium acetate.

7. An electrolyte for use in the method defined in claim 1 whichconsists essentially of an aqueous solution of an alkali-metaldichromate, an alkali-metal acetate in an amount of at least 5 and atmost [0 grams per 10 liter, and chromium trio xide in an amount of atleast 4 and at most 6 grams per liter, said electrolyte having ahydrogen-ion concentration corresponding to a pH between 40 and 5.0 andan electrical conductivity of at least 28,000 micromhos per centimeterat a temperature of 50C.

8. A method as defined in claim 1 in which the electrolyte consistsessentially of an aqueous solution of an alkali-metal dichromate in anamount of at least 20 and at most 25 grams per liter, an alkali-metalacetate in an amount of at least 5 and at most 10 grams per liter, andchromium trioxide in an amount of at least 4.5 and at most 5.5 grams perliter, the said electrolyte having an electrical conductivity of atleast 30,000 micromhos per centimeter at a temperature of 50C.

1. A METHOD FOR TREATING A MATERIAL HAVING A TIN-CONTAINING SURFACE TOPASSIVATE AND IMPROVE THE CORROSION RESISTANCE OF THE SAID SURFACE WHICHCOMPRISES MAKING THE MATERIAL A CATHODE IN AN AQUEOUS HEXAVALENTCHROMIUM ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF ANALKALI-METAL DICHROMATE IN AN AMOUNT OF AT LEAST 20 AND AT MOST 25 GRAMSPER LITER, AN ALKALI-METAL ACETATE IN AN AMOUNT OF AT LEAST 5 AND ATMOST 10 GRAMS PER LITER, AND CHROMIUM TRIOXIDE IN AN AMOUNT OF AT LEAST4 AND AT MOST 6 GRAMS PER LITER, THE SAID ELECTROLYTE HAVING AHYDROGEN-ION CONCENTRATION CORRESPONDING TO A PH BETWEEN 4.0 AND 5.0 ANDAN ELECTRICAL CONDUCTIVITY OF AT LEAST 28,000 MICROMHOS PER CENTIMETERAT A TEMPERATURE OF 50*C, AND PASSING AN ELECTRIC CURRENT THROUGH THEMATERIAL AND THE ELECTROLYTE UNTIL AN AMOUNT OF CHROMIUM-CONTAINING FILMIS DEPOSITED UPON THE SAID SURFACE THAT IS SUFFICIENT TO IMPROVE THECORROSION RESISTANCE OF THE SAID MATERIAL
 2. A method as defined inclaim 1 in which the electric current is passed through the material andthe electrolyte for a period between 1 and 2 seconds and the saidcurrent has a density as measured at the cathode of at least 5.5 and atmost 9.5 amperes per square decimeter, so that between 8 and 14 coulombsper square decimeter of current are consumed.
 3. A method as defined inclaim 2 in which the electrolyte is maintained at a temperature between30* and 60*C.
 4. A method as defined in claim 2 in which the electrolyteis maintained at a temperature between 40* and 50*C.
 5. A method asdefined in claim 2 in which the electric current is passed until theamount of chromium-containing film that is added to the material isequivalent to at least 25 milligrams of chromium per square meter ofmaterial.
 6. A method as defined in claim 1 in which the alkali-metalacetate is sodium acetate.
 7. An electrolyte for use in the methoddefined in claim 1 which consists essentially of an aqueous solution ofan alkali-metal dichromate, an alkali-metal acetate in an amount of atleast 5 and at most 10 grams per liter, and chromium trioxide in anamount of at least 4 and at most 6 grams per liter, said electrolytehaving a hydrogen-ion concentration corresponding to a pH between 4.0and 5.0 and an electrical conductivity of at least 28,000 micromhos percentimeter at a temperature of 50*C.
 8. A method as defined in claim 1in which the electrolyte consists essentially of an aqueous solution ofan alkali-metal dichromate in an amount of at least 20 and at most 25grams per liter, an alkali-metal acetate in an amount of at least 5 andat most 10 grams per liter, and chromium trioxide in an amount of atleast 4.5 and at most 5.5 grams per liter, the said electrolyte havingan electrical conductivity of at least 30,000 micromhos per centimeterat a temperature of 50*C.